(function (doc, win) {  // rem 響應式
  var docEl = doc.documentElement,
    resizeEvt = "orientationchange" in window ? "orientationchange" : "resize",
    recalc = function () {
      var clientWidth = docEl.clientWidth;
      if (!clientWidth || clientWidth < 1600) {
        docEl.style.fontSize = '20px';
        return false;
      }
      docEl.style.fontSize = 20 * (clientWidth / 1600) + "px";
    };
  recalc();
  if (!doc.addEventListener) return;
  win.addEventListener(resizeEvt, recalc, false);
  doc.addEventListener("DOMContentLoaded", recalc, false);
})(document, window);